The present invention particularly relates to a flame atomic absorption spectrophotometer of the type which requires air as a start-up and shut-down oxidant and uses nitrous oxide as a high energy oxidant for the burner flame.
In atomic absorption spectroscopy, the measurement of the absorption of a radiation beam at a characteristic resonant spectral line for a particular element yields a measure of the concentration of that element in an original sample solution. At the present time, one of the most common techniques for atomizing an element for purposes of the absorption measurement is by introducing a liquid sample solution of the element of interest into a gas burner wherein droplets of the solution are vaporized and the elements ultimately atomized, so as to form in the path of the apparatus radiation beam, a substantial quantity of the element of interest in its atomic state. A sample light beam, which originates from a line-emitting light source, and which includes a resonance line of the element to be measured, is directed through the flame. The desired element in the sample absorbs the resonance lines characteristic of the element and the emerging light beam is directed to a monochromator and thence to a detector which measures the degree to which the desired element absorbs the resonance lines of the sample beam. This absorption degree represents the amount of desired element in the sample substance.
In such spectrophotometers, in order to produce a flame which has a high enough temperature for the best measurement results for certain elements, it is preferred to use acetylene gas as a fuel and to use nitrous oxide (N.sub.2 O) as the source of oxygen for the combustion of the acetylene gas. In order to initiate combustion in a safe manner, it is necessary to begin combustion of the acetylene gas using air as the oxygen source, and to then switch over to the nitrous oxide after the acetylene gas flame is ignited and stable.
It is also a characteristic of such a system that, in order to shut down the system in a safe manner which avoids the risk of explosions, it is customary to change back from nitrous oxide as the oxidant to air as the oxidant before turning off the burner system. This has always necessitated the provision of some means for assuring that there would be a source of air under pressure available when shut-down of the system is to be effected. Otherwise, failure of the air supply system leaves the operator with no safe means of shut-down. One common procedure for assuring the presence of an emergency air supply is to provide an accumulator container of compressed air which is kept in reserve for such a contingency. However, the necessity for an accumulator creates considerable added expense and complexity, and an increase in the size of the apparatus.
Accordingly, it is one object of the present invention to provide for a safety shut-down of the flame of the spectrophotometer without requiring the expense and complication of a compressed air accumulator container.
Another problem in accomplishing a safe shut-down of the system arises if there is a failure of electrical power. This is a problem because the spectrophotometer requires electrical power for normal productive operation, and should be shut down if the power fails. Furthermore, the controls for the burner system are electrically actuated, particularly the normal shut-down routine controls which shift the operation of the system from nitrous oxide back to air as the oxidant, and then shut the system down from that condition. Accordingly, in present flame atomic absorption spectrophotometer systems, it is necessary to provide for at least a short period of maintenance of power on the system in case of main power failure. Such power maintenance must be provided by means of standby batteries, or by means of large storage capacitors which are capable of maintaining power for a period sufficient to accomplish a safe shut-down.
Accordingly, it is another important object of the invention to provide for a safety shut-down of the flame of the spectrophotometer in case of a failure of electrical power for the spectrophotometer system.
A further object of the invention is to provide for a safety shut-down of the flame of the spectrophotometer in case of a simultaneous failure of electrical power and air pressure for the spectrophotometer system.
Further objects and advantages of the invention will be apparent from the following description and the accompanying drawings.